(1) Field of the Invention
This invention relates to paving compositions consisting of a mixture of aggregate and bituminous emulsions. Generally, there are two ways to produce such a mixture which, after application onto a worn out road surface, results in a strong matrix suitable to bear heavy traffic. One way to produce an aggregate asphalt emulsion mix is to apply emulsion onto the road surface to be repaired and spread aggregate on top. The stones, which make up the aggregate, sink into the emulsion; and upon setting of the emulsion, which is the result of evaporation of the water from the emulsion, the stones are locked into place; and a strong matrix is obtained. Emulsions used for this type of application are generally referred to as rapid setting emulsions. The main specification for anionic rapid setting emulsions according to ASTM-D977 is a minimum demulsibility of 60% when treated with a 0.02N CaCl.sub.2 solution. The second method to achieve an aggregate-emulsion mix is by mixing the required amount of aggregate and asphalt emulsion at a separate location in a central mixing plant. These mixes are hauled to the locations where they are applied. One important feature of the asphalt emulsions to be suitable for this technique is the ability to produce a mix which still can be handled after transport to the location of application. The most suitable way to do this is to use cutback asphalt from the emulsion. Cutback asphalt is generally referred to as a blend of fuel oil or other petroleum distillates and asphalt. The content of oil may be in the range of 1-15% based on asphalt. The advantage of using cutback asphalt for these mixes is the fact that even when the emulsion in the mix breaks prematurely, the viscosity of the asphalt is still low enough that the mix can be handled. Emulsions used for this type of application are referred to as medium setting emulsions. According to ASTM-D977, they are distinguished from rapid setting emulsions in that no minimum demulsibility is required.
Both cationic and anionic emulsions are applied in the field. They distinguish themselves by the type of the emulsifier which is used for the preparation and especially by the pH value of the emulsions. Cationic emulsions are generally prepared at pH values between 1.5 and 7.0. Nitrogen containing chemicals, such as fatty amines, fatty diamines, and fatty amidoamines, fatty quaternary-amonium salts or aminolignins, are used as emulsifiers. Anionic emulsions are prepared at pH values ranging from 7 to 12. Fatty acids or fatty sulfonic acids, which upon treatment with alkali, such as sodium hydroxide or potassium hydroxide, form anionic soaps, are the most effective emulsifiers.
One feature of bituminous pavements regarded as objectionable is their tendency to soften in hot weather resulting in shoving, rutting or bleeding (migration of the binder to the surface) of the matrix. Hard asphalts are not softened at increased temperatures; but they are hard to emulsify and show objectionable properties at low temperatures, such as premature cracking.
(2) Description of the Prior Art
By employing a suitable emulsifier system for the preparation of the asphalt emulsion, soft asphalts can be modified in a way that after the emulsion has set, the asphalt will be modified so that the temperature range where it possesses decreased flow is extended. Low temperature susceptibility is also improved by maintaining its ductile properties in cold weather (less brittle at low temperature). In general terms, by this modification of the asphalt, the deformation of the road surface in hot weather is prevented, resulting in loss of aggregate from the surface. Emulsifiers, where the hardness at higher temperatures of the asphalt is increased and the hardness at lower temperatures is not adversely affected, are disclosed by K. E. McConnaughay in U.S. Pat. No. 2,855,319. The most effective emulsifying agent is tall oil soap, obtained as the by-product of the kraft pulping process whereby chipped pinewood is digested with sodium hydroxide and sodium sulfide. If crude tall oil is employed to prepare the emulsion, generally 2% to 4% based on the weight of asphalt is sufficient to obtain high float emulsions. Both increased melting point and resistance to flow at elevated temperature are measured by the "float test." In this test (ASTM-D139) a plug of "emulsion residue" is solidified in a brass collar by an aluminum float which is placed into a water bath heated to 140.degree. F. The time required for the water to break through the plug is determined. The asphalt emulsion residue forms the high float test when the collar floats for at least 1,200 seconds. The "residues" of the emulsion typically are considered to be what remains from an asphalt emulsion after the water is evaporated. The significance attached to the high float property is that the residue from a high float emulsion will not flow under the force of gravity at temperatures as high as 140.degree. F. As a result of the high amounts of tall oil soap emulsifier, these high float emulsions show the characteristics of medium setting emulsions since the demulsibility with calcium chloride solution is less than 60% (in most cases less than 20%), which minimum value is specified for rapid setting emulsions (ASTM-D977). "Rapid setting" emulsions are useful for seal coats, surface treatments, penetration treatment of macadam, etc. The preparation of rapid setting high float emulsions is disclosed by L. F. Ostermeyer et al. in U.S. Pat. No. 4,433,084. In one embodiment of the Ostermeyer et al. process, asphalt cement is mixed with tall oil or tall oil derivatives, such as tall oil pitch or black liquor soap skimmings, at levels of 1-5% based on the weight of the blend prior to emulsification. The blend is then emulsified with accepted techniques to provide a rapid setting emulsion. According to a second embodiment, tall oil pitch or derivatives (such as tall oil heads, tall oil fatty acid, or rosin) are reacted with strong base, such as sodium hydroxide or potassium hydroxide, to form a suitable modifier which is added to the asphalt cement prior to emulsification at a dosage of 0.4-10% based on the weight of the asphalt. In a third embodiment, an alkaline soap of tall oil pitch (or derivative) is formed and added to the asphalt cement prior to emulsification. Rapid setting high float emulsions are also known to be obtained by incorporating 1-5% of a styrene-butadiene-styrene block copolymer or styrene-isoprene-styrene block copolymer into the asphalt cement prior to emulsification.